Incandescent filament



Dec. 10, 1963 H. B. SLOAN 3,113,393

INCANDESCENT FILAMENT Filed Oct. 14. 1960 FIG. I

CARBON COATING HEATED COlLED-COIL TANTALUM FILAMENT GAS CONVECTION CU RRENTS COIL FILAMENT OF TANTALUM PAINTING UPPER HALF OF RESULTANT WITH COLLOIDAL GRAPHITE MOUNTING COIL IN LAMP ENVELOPE HEATING COIL BY PASSING CURRENT THROUGH IT IN CARBURIZING ATMOSPHERE SEVERAL TIMES IN SERIES OF STEPS EXHAUSTING LAMP ENVELOPE FILLING LAMP ENVELOPE WITH FINAL FILLING GAS HOWARD B. SLOAN Fl 2 INVENTOR BY W M ATTORNEY United States Patent 3,113,893 INCANDESCENT FILAMENT Howard B. Sloan, Topsfield, Mass, assigncr to Sylvania This invention relates to incandescent lamps. Such lamps generally comprise an electrically-incandescible body, such as a filament, in a sealed enclosing envelope of a light-transmittingmaterial such as glass.

The incandescible body has generally been of tungsten, the brightness and efficiency of the lamp being therefore no greater than that obtainable at the melting point of tungsten, which is about 3300" C.

A much higher brightness can be obtained by maldng the incandescible body of a more refractory material, but if it is to be heated by passage of current through it, the material must be also electrically conductive. Tantalum carbide is a conductive material more refractory than tungsten and has been suggested for use as an incandescible body in an incandescent lamp.

Tantalum carbide, however, is hard and bri-tle, so it cannot be drawn into wire, and even if it could be so drawn, would be practically impossible to form into a coil.

The suggestion has therefore been made that a coiled filament be made of tantalum metal, mounted on suitable support and lead-in wires in a lamp envelope, and then transformed to tantalum carbide in situ by being heated to a suitable temperature in the presence of a carburizing gas, generally a hydrocarbon such as methane. A small amount of the latter is preferably used in an atmosphere of another gas such as hydrogen.

The filamentary coil will be distorted considerably during the carburizing process, particularly if it is a so-called coiled-coil, that is, a doubly coiled filament. The distortion will be in the form of a tangling of the Wires, in addition to a sag in the coil.

The distortion can be reduced by performing the carburization in a series of steps, preferably with a fresh atmosphere of carburizing gas being used for each step.

Although other distortions will be reduced by the step carburizatio-n, the filament will still sag seriously at some time before the carburization is completed. I have discovered, however, that the sagging can be almost entirely prevented by coating the coil with graphite, for example, by painting it with a colloidal suspension of graphite in water, such as that known as Aquadag, for example. If the axis of the coil is horizontal, the sag will be reduced most effectively if the graphite is aplied only to the top half of the coil, that is to the coil above the horizontal plane containing its longitudinal axis.

The remarkable effect from so coating the top portion of the coil is believed to be due to the convection currents in the carburizing gas. The gas rises through the heated coil, from bottom to top, and becomes somewhat depleted of carbon before reaching the top of the coil. The graphite coating of the coil insures proper carburizing of the top of the coil, by providing additional carbon.

Other objects, advantages and features of the invention will be apparent from the following specification, taken in connection with the accompanying drawings, in which:

FIG. 1 is a schematic drawing of a filament coil coated with carbon and being heated in a carburizing atmosphere; and

FIG. 2 is a flow diagram indicating the order of steps in the method.

In performing one embodiment of the invention, 1 start with a tantalum filament whcih can be a coiled-coil of 0.007 inch diameter material, the minor coil having ice 2 an inside diameter of 0 .010 inch, and the major coil an inside diameter of 0.030 inch. The coil can be of tantalum or an alloy thereof, for example, Tantaloy, an alloy of tantalum with about 7.5% tungsten.

The filament is made by doubly-coiling a wire of metallic tantalum, or suitable alloy thereof, then welding it to the support wire ends 3, 4 mounting it in a lamp envelope and then carburizing it by passing enough current through it to raise it to a temperature between about 2 000" C. and 2600 C. in an atmosphere of hydrogen containing about /s of 1% to A of 1% of methane gas, the total gas pressure being about atmospheric, that is, about 800 mm. of mercury. However, we prefer to use hydrogen saturated with xylol at room temperature, xylol being a hydrocarbon.

During the carburizing, the coil increases about 24% in volume and this tends to distort the shape of the coil considerably especially when the filament is a coiled-coil. The distortion can be reduced by carburizing in a series of steps. For example, the coil can first be heated at about 2250" C. for about 1 /2 minutes, then pumped out, that is, exhausted of gas, and refilled with the same gas mixture. The gas mixture used can be 100 mm. of hydrogen saturated with xylol, as above, and 700 mm. of reasonably pure hydrogen.

The above heating, exhausting and filling process can be repeated about five times, and then the gas filling can be changed to 800 mm. of mercury pressure of hydrogen saturated with Xylol at room temperature, the hydrocarbon content being thus increased. The temperature is then raised to 23 C. for about half a minute, and then raised slowly to 2500 C. in about one and one half minutes. This is repeated about four times, and then the lamp can be exhausted and filled with the final filling gas that is to be present in the lamp. This can be the gas filling previously mentioned.

The above process will result in a tantalum carbide coil with less distortion than that obtained with quick, one-step conversion. However, the coil will still sag somewhat before the fifth step is reached.

The sagging can be substantially entirely prevented if the coil is to be carburized in a horizontal position by painting the upper half of the coil surface with a suspension of colloidal graphite in water, for example, the suspension lcnown as Aqu-ad-ag. The effectiveness of this coating is apparently due to the depletion of the carbon in the hydrocarbon gas as the latter rises from the top to the bottom of the hot coil by convection. Whatever be the reason, the etfect of applying the carbon is dramatic. It completely ends the sagging.

The above is particularly true of coils in which the minor and major windings are in the same direction, for example, both clockwise. Coiling the major and minor windings in opposite directions appears to enhance the sagging.

For the filament described, made of a wire about 132 mm. in length, about 1.4 mg. of graphite was used on the coil.

By an incandescible material in the foregoing specification is meant one which can be made incandescent.

The amount of graphite used on the upper half of the filament described was 1.4 mg.

The temperatures given in the specification were measured on an optical pyrometer.

What -I claim is:

1. The method of forming a coiled metal carbide filament coil which comprises the step of coating a metal filament coil with graphite and then heating the coil to carburize the same.

2. The method of claim 1, where the metal is tantalum.

3. The method of forming a coiled metal. carbide filament coil which comprises the step of coating the top only of a metal tfilament coil with graphite and heating the coil to convent the metal to a carbide.

4. The method of fiorming a coiled metal carbide filament coil which comprises the step of coating the top only of a metal filament coil with graphite and heating the coil in an atmosphere of hydrocarbon to convert the metal to a carbide.

5. \The method of forming a coiled tantalum carbide filament which comprises the step of coating the top of a coiled tantalum filament with graphite, and then heating the coil in an atmosphere of hydrogen to convert the tantalum to tantalum carbide.

6. The method of forming a metal carbide filament coil which comprises heating a metal filament coil to a high temperature in an atmosphere of hydrocarbon, removing the hydrocarbon atmosphere used in the previous step, placing a new atmosphere of hydrocarbon around the filament coil, and again heating the filament to a high tempenature, and repeating the process several times.

7. The method of claim 6, in Which the filament coil is first coated with graphite.

8. The method of forming a metal carbide filament coil which comprises heating a coiled tantalum filament to a temperature of about 225 0 C. and raising it to about 2500 C. in a minute and one half in an atmosphere of hydrocarbon and hydrogen, removing the atmosphere used in the previous step, substituting a new atmosphere of hydrocarbon and hydrogen, reheating the tantalum filament coil in the same manner, repeating these steps several times, then repeating the steps several times more but at a temperature higher than said first-mentioned temperature.

References Cited in the file of this patent UNITED STATES PATENTS Germany July 3, 1958 

1. THE METHOD OF FORMING A COILED METAL CARBIDE FILAMENT COIL WHICH COMPRISES THE STEP OF COATING A METAL FILAMENT COIL WITH GRAPHITE AND THEN HEATING THE COIL TO CARBURIZE THE SAME. 